The hygiene hypothesis postulates that exposure to microbes early in life provides critical stimuli that drive immune system maturation away from the development of allergic susceptibility. The objective of this competing renewal is to investigate three immunologic pathways that appear to protect against asthma, to determine whether these pathways provide a mechanistic basis for the hygiene hypothesis, and to determine how asthma susceptibility genes may interact with environmental influences to protect against asthma. The specific aims of the project are to: (1) determine the relation between the capacity to produce IFN-gamma in infancy and the risk for the subsequent development of asthma, and to identify any influence of microbial exposure or single nucleofide polymorphisms (SNPs) in pattern recognition receptors on that development; (2) establish the influence of IL-10 production in early life on the ratio of allergen specific IgG4/IgE and the subsequent development of asthma, and to determine the influence of the innate immunity genes TLR4 and CD14 on IL-10 production; (3) establish the biologic basis for the decreased prevalence of asthma associated with a particular SNP in TLR4; and (4) assess the influence, separately and combined, of TLR SNPs and household microbial exposure on peripheral blood monocyte expression of TLRs in early life. The project builds on the successful, on-going Infant Immune Study, which is following a birth cohort to 8 years of age. The competing renewal will enroll the last third of the population of infants required for adequate statistical power, will newly assess early microbial exposures, allergen specific IgG4, and TLR expression, and will follow already enrolled subjects to age 8. Blood is obtained for DNA and immunologic assessments (cytokine production, total IgE, allergenspecific IgE and IgG4) six times in the first 5 years of life; blood is also drawn on parents. Skin prick tests are administered to parents and, at age 5, to the children. Respiratory questionnaires are obtained frequently to determine onset and recurrence of asthma symptoms, and medical record reviews conducted to identify diagnosis of asthma and other respiratory conditions. Dust samples are obtained from the home in early infancy to assess exposure to endotoxin, and Gram-negative, Gram-positive and total bacteria. This project integrates epidemiologic, immunologic, genetic, and molecular approaches to study, in a comprehensive manner, how environmental factors and genetic susceptibility interact to alter the maturation of the immune system and subsequent risk of asthma. An understanding of these processes is critical to elucidating the pathogenesis of asthma, and the development of effective strategies for the primary prevention of asthma.